The present invention relates to an ink-transfer thermal printer, and, more particularly, to an improved head driving and ink ribbon feeding mechanism for an ink-transfer thermal printer.
The use of non-impact printers as medium and low speed printer output terminals of electronic business machines in business offices is becoming increasing prevalent, a large factor of their appeal being the silent printing operation afforded thereby, particularly in comparison with conventional impact printers. At present, ink-jet printers and thermal printers comprise the two major types of such non-impact printers.
As is well known, ink-jet printing is performed by ejecting micro-ink drops from micro-nozzles of an ink-jet head toward the recording paper; the micro-nozzles of ink-jet printers, however, are known to present maintenance problems.
Thermal printers, utilizing thermal energy as the basic printing method or technique, avoid the maintenance problem of ink-jet printers but introduce yet other problems. In this regard, there are two basic categories of thermal printing techniques.
The first thermal printing technique is known as thermal-sensitive printing, and employs a thermographic paper coated with a thermo-sensitive coating which, when heated above some predetermined minimum temperature, undergoes a color change. The thermographic paper employed in such thermo-sensing printing, however, is rather costly and the printed images tend to fade or become discolored over time.
A second category of thermal printing is known as ink-transfer thermal printing, wherein a thermo-ink, coated as a layer on a base ribbon of plastic film, is transferred selectively to the recording paper in accordance with the images or characters to be reproduced. The thermo-ink is solid at room temperature, but changes rapidly to a softened or molten state above a predetermined temperature. A thermal head, which may be of a dot matrix variety, is rapidly heated and caused selectively to contact the thermal-ink ribbon against the recording paper at a predetermined pressure, such that the selectively heated printing elements arranged on the thermal head cause the corresponding, contacted areas of the thermo-ink to become softened or molten and to selectively transfer from the ribbon to the recording paper as spots, or dots, of the thermo-ink material, the transferred dots providing a durable printed image on the recording paper. The thermo-ink is usually a carbon black powder or pigments which are mixed with a binder such as wax. As is readily apparent, the thermo-ink ribbon can be used only once in a given area, because the selective transfer of dots of the thermo-ink to the recording paper necessarily leaves the corresponding portions of the base ribbon exposed, or depleted of further thermo-ink material. Since it is somewhat expensive, the thermo-ink ribbon should be used as efficiently as possible; particularly, and by way of example, during spacing operations in which no printing is performed, the thermo-ink ribbon should not be fed or advanced, since otherwise the advanced portion of the thermo-ink ribbon is needlessly consumed and thus wasted. Accordingly, it is important that a thermo-ink ribbon feeding mechanism be designed to be coordinated with the movement of the thermal head and the circumstance of whether printing is or is not to be performed at any given position in which the print carriage is moved; particularly, the ribbon should be advanced by the required pitch (i.e., of the next print function, whether a full character or a column of selected dot positions) when printing is to be performed, and alternatively it should not be advanced when no printing is to be performed (i.e., as in a spacing operation).
Mechanisms have been proposed in the prior art for the purpose of achieving this efficient use of the thermo-ink ribbon and particularly to avoid advancing same during carriage movement for a spacing operation in which no printing is to be performed. FIG. 1 is a schematic, perspective view of such a prior art ink-transfer type thermal printer, FIG. 2 comprising a cross-sectional elevational view of the thermal printer of FIG. 1 illustrating the printer carriage mechanism and associated elements. With concurrent reference to FIGS. 1 and 2, recording paper 7 is wrapped partially about a cylindrical platen 6 and advanced thereby to receive successive lines of print, as desired, along the horizontal, printing direction illustrated by the line x--x in FIG. 1, the line x--x extending generally parallel to the axis of the cylindrical platen 6.
In typical operation, following printing on a given print line, the recording paper 7 is advanced in the direction of the arrow a by a corresponding, intermittent rotation of the platen 6; typically, guide means (not shown) are provided to guide the paper 7 during this advancing operation. A carriage 5 is mounted in sliding engagement on a main guide bar 8 for selective translational movement therealong, in the alternative or opposite directions shown by the arrows b and c, parallel to the axis of the platen and thus to the printing direction x--x. Carriage 5 is driven by a carriage feeding means, typically comprising driving cables with related pulleys and a driving motor (not shown), for the selective translational or sliding movement along main guide bar 8. Carriage 5, moreover, is selectively rotatable about the main guide bar 8 in the alternative or opposite direction shown by the arcuate arrows f and g.
A thermo-ink ribbon 1 extends from a supply spool thereof (not shown) across the face of the cartridge 3, such that a length of the ribbon 1 is disposed adjacent the current print line x--x on the recording paper 7, and is advanced onto a take-up reel 2 which, as seen in FIG. 2, is received over and driven by a shaft 9. Shaft 9 is mounted on the carriage 5 for rotary motion and extends downwardly therefrom, carrying at its lower end a roller 10 affixed thereto for rotatably driving the shaft 9.
Carriage 5 supports a thermal head 4 which may be of conventional type, the ribbon 1, as best seen in FIG. 2, being interposed between the thermal head 4 and the recording paper 7 which in turn is wrapped about the platen 6.
A head driving mechanism for the carriage 5 comprises an electromagnetic solenoid 11 and associated plunger or spindle 13, a horizontal bar 12 coextensive with and extending parallel to the main guide bar 8, and the aforenoted take-up spool drive shaft 9 and associated roller 10. The horizontal bar 12 is mounted for generally horizontal, reciprocating movement selectively in the opposite directions indicated by the double-head arrow e in FIG. 2. Particularly, the bar 12 is to be driven to the right (i.e., as seen in FIG. 2) by the spindle 13 of the solenoid 11 when the latter is energized, to rotate the carriage to the so-called "head-down" position in which the thermal head 4 selectively engages the ribbon 1 against the recording paper 7 to perform a printing operation; conversely, spring means (not shown) normally bias the carriage 5 to rotate in the direction of arrow g, returning same to the so-called head-up position when the solenoid 11 is not energized.
More specifically, when the solenoid 11 is energized, the spindle 13 pushes the horizontal bar 12 in a rightward direction as seen in FIG. 2, transversely to the axis of the platen 6, which operates in turn through the roller 10 and shaft 9 to rotate the carriage 5 in the direction of the arrow f shown in FIG. 2, and thus counterclockwise about the main guide bar 8, thereby rotating the thermal head 4 toward the thermo-ink ribbon 1 and the recording paper 7. The thermal head 4 thus engages the thermo-ink ribbon 1 and the recording paper 7 against the platen 6 with a predetermined pressure. This rotaty motion of the thermal head 4 in the direction f is referred to as a "head-down" operation.
In known manner, thermal printing elements (not shown) are arranged on the face of the thermal head 4 in a vertical line, perpendicular to the print line x--x. When the thermal head 4 is pressed against the thermo-ink ribbon 1 and the appropriate printing elements are selectively heated by their respective heaters, the corresponding, softened or molten portions of the thermo-ink layer are transferred to the recording paper 7, leaving a dot pattern thereon. Thereafter, the heaters for the selective elements are turned off. The roller 10 is engaged with the guide bar 12 and the carriage 5 is advanced in translational movement along the main guide bar 8 in the direction of the arrow b, thereby rotating the roller 10 in the direction of the arrow d as a result of the frictional engagement of the roller 10 with the horizontal bar 12. This in turn rotates the shaft 9 of the take-up reel 2 mounted within the cartridge 3, advancing the thermo-ink ribbon 1 and winding up the used thermo-ink ribbon 1 onto the take-up spool 2. Thus, a fresh portion of the thermo-ink ribbon 1 is positioned in front of the thermal head 4, in preparation for the next printing operation.
When a space is designated in a line of print, the solenoid 11 is de-energized and simultaneously the plunger 13 and the horizontal bar 12 are withdrawn by spring means (not shown), thus disengaging bar 12 from the roller 10. Particularly, the carriage 5 is rotated about the main guide bar 8 in the direction of the arrow g (i.e., clockwise in FIG. 2) by springs (not shown) to the "head-up" position illustrated in FIG. 2. In the "head-up" position, the thermal head 4 is spaced apart from the thermo-ink ribbon 1; moreover, since the bar 12 is spaced from the roller 10 of the shaft 9, the take-up reel 2 is not driven and thus the thermo-ink ribbon 1 is not advanced during subsequent translation of the carriage 5 along the main guide bar 8, thus completing a space operation. Since the thermo-ink ribbon 1 is not advanced during the space operation, improved economy is achieved since the expensive thermo-ink ribbon 1 is not wasted by being advanced during the space operation, thus decreasing the operating costs of the thermal printer.
The prior art mechanism for an ink-transfer thermal printer head, as illustrated in FIGS. 1 and 2 and described above, will be seen to perform a mechanical switching operation, for moving the thermal head 4 between the head-down and head-up states or positions. The mechanism is of substantial mass and includes the entirety of the printing carriage 5, a thermo-ink ribbon cartridge including the ribbon and its supply and take-up spools, and the shaft 9 and roller 10 for driving the latter; likewise, the lengthy horizontal bar 12 must be moved for each switching operation. In view of the mass of these moveable members, it is difficult for such prior art mechanisms to perform the above-described mechanical switching operations rapidly, thus limiting the printing speed of the printer. Moreover, the electro-magnetic solenoid operation and the related mechanical engaging functions involving the horizontal bar 12 produce substantial operating noise, detracting from the advantage of the "silent" printing operation of the thermal printer itself.
Various other carriage drive and thermo-ink ribbon winding mechanisms are known for thermal head printers. One such prior mechanism, disclosed in Japanese Laid-Open Patent Application TOKU-KAI-SHO No. 57-92180, published June 7, 1982, employs a small rocking arm which is mounted pivotally on the carriage and carries the thermal head on a free end thereof. As in the above described prior art structures, the carriage is mounted for sliding, translational movement along a guiding means, parallel to the platen. However, the carriage is not actuated in rotary fashion as aforedescribed, and instead it remains stable and only the rocking arm and thermal head are moved during printing operations. By thus minimizing the mass of the moveable elements, rapid head-down and head-up switching operations are possible permitting high-speed printing operations. Whereas high speed operation is obtained with this mechanism, the feeding of the thermo-ink ribbon and the transportation of the thermo-head cartridge are controlled independently, resulting in a somewhat complicated mechanism which also is costly to produce.